The present invention relates to an electrostatic lens for electron microscopes, focussed ion beam apparatuses, or the like.
Electrostatic lenses have been used as focussing lenses for electron guns, focussed ion beam apparatuses, or the like. In place of an electromagnetic lens of an electron microscope, an electrostatic lens is expected to be used as a focussing lens for apparatuses using electron beams, particularly for semiconductor device inspecting apparatus. The reason for this is that electrostatic lenses can be made compact and light, and in addition, an improved aberration characteristics in the low acceleration voltage range can be expected.
The aberration characteristics in the low acceleration voltage range are required to be improved because it is necessary for semiconductor devices to be protected from damage by electron bombardment and it is necessary to prevent electric charges from being accumulated.
Electrostatic lenses have to meet the following requirements.
First, it is necessary to reduce the accumulation of electric charge. Within the structure of an electrostatic lens assembly, there exist not only electrons passing through a normal orbit along the optical axis, but also stray electrons dispersed by an aperture and the like. These stray electrons are piled up on the inner wall of a holder and charge it up. Therefore, an electric field which should not be otherwise present is generated, lowering the focussing effects of the lens and making the electron orbit unstable.
This phenomenon becomes particularly conspicuous when the energy of electrons is low. Many electrostatic lenses are being used to inspect semiconductor devices. In order to not damage semiconductor devices during inspection, it is necessary to use electrons of a low energy and solve the problem of electric charge accumulation.
Secondly, it is necessary that a holder for holding a lens should not contaminate specimens. In many cases, as the holder material, material generally called machinable ceramic having good machinability is used.
However, this machinable ceramic has many impurities. These impurities are emitted in the form of gas within the vacuumed holder, and attach to a specimen and contaminate it. Generally the holder is baked in order to raise the degree of vacuum. Therefore, the phenomenon of gas emission of impurities is accelerated. As described above, an electrostatic lens is often used in inspecting semiconductor specimens, and so such contamination poses a significant problem.
Thirdly, it is necessary to reduce the optical aberration of a lens. The most important parameter in evaluating the lens performance is an aberration coefficient. An aberration of an electrostatic lens generates locally, and the aberration at this local area determines the whole lens performance.
It has been proposed to smooth a potential change by using a number of lens electrodes and applying voltages having a small difference therebetween to the lens electrodes (Reference Document: M. Szilagyi and L. Szep, "Optimum design of electrostatic lenses", J. Vac. Sci. Technol., B6(3), May/June 1988, pp. 953-957).
However, use of a number of lenses increases the number of electrodes. The lens assembly described in the above Document uses twelve electrodes. There arises the problem of a complicated structure, increased manufacturing cost, and poor utility.
It has been also considered to use a lens of a larger inner diameter to smooth the potential change. However, with a large inner diameter, the outer dimension of an electron optical barrel becomes large, which is quite a disadvantage from the viewpoint of an object of using an electrostatic lens which is essentially smaller than a conventional electromagnetic lens.